Sulfur compounds in petroleum present an enormous variety of chemical structures and are described by chemical classes (thiols, disulfides, sulfides, thiophenes, etc.). The relative stability of various sulfur species are considerably different; sulfides, disulfides and thiols in the petroleum feed stocks are more easily desulfurized during desulfurization processes on a refinery than the aromatic sulfur compounds. Compounds containing sulfur atom in an aliphatic structure (e.g. thiols, sulfides, disulfides) are more reactive (hence more corrosive) than compounds with the sulfur atom in an aromatic ring structure (e.g. thiophenes, benzothiophenes, dibenzothiophenes, etc.). Therefore, the sulfur compounds in the product stream of a desulfurization process are mostly in aromatic form and are considered more unsusceptible (non-reactive sulfur) to desulfurization processes. Among these, the substituted benzothiophenes and dibenzothiophenes are the most difficult to remove compared to unsubstituted thiophenes, benzothiophenes, or dibenzothiophenes. It is also well recognized by industry that each of these classes contributes differently to corrosion at high temperatures in a refinery. Thus, determination of the corrosive behavior of oils requires an assessment of sulfur containing organic compounds that it contains. The term “reactive sulfur” can be defined as the organic sulfur that generates H2S under thermal stress or relatively easy to remove by desulfurization. Research indicates that such compounds (non-aromatic mercaptans, sulfides, and disulfides) are corrosive in refinery stress at temperatures between 250-400° C. (450-700° F.). Thermally stable organosulfur compounds (thiophenes, thiophenols, and aryl sulfides where sulfur electrons are conjugated with the aromatic ring) are termed “‘non-reactive”. It will be recognized by those familiar with refinery operations that the “corrosive” sulfur classes are also the most reactive in common refinery processes such as hydrotreating, cracking or coking while the “non-corrosive” thiophenic are the least “reactive”. Thus, the instant invention applies equally to the distinction of reactivity. This application will use the terms “reactive” and “non-reactive” with the understanding that the alternative corrosiveness designation applies equally well.
Thiophenes are generally considered to be the dominant form of organic S in petroleum fractions boiling in the critical temperature range; consequently, total sulfur analyses do not reflect the concentration of reactive sulfur. Indigenous H2S and elemental S are measured by existing routine techniques, if necessary.
Cataldi (1953) proposed to measure “corrosive potential” by determination of H2S evolution at 800° C. Alternatively, Drushel (1956) proposed measuring total S before and after high temperature pyrolysis over a catalytic surface to determine the “thiophenic” and “non-thiophenic” sulfur. Piehl (1960) expanded on this concept by measuring the H2S evolution while heating crudes in glass at 2° F./min. Other research has suggested that chemisorption of corrosive sulfur compounds onto metal surfaces initiates H2S evolution at lower temperature.
As an alternative to H2S evolution, it has been proposed that oils be characterized by a combination of different analytical techniques. In 1974, an API study group reported a scheme wherein oils have been characterized by suite of analytical procedures for the determination of all classes of S (S, H2S, mercaptans, sulfides, disulfides, and thiophenes by difference). Various refinements of these techniques have evolved for the measurement of specific classes especially for the measurement of sulfides. In particular, techniques have been developed based on iodine complexes and UV measurement, oxidation coupled FTIR measurement, oxidative titration to end points, XPS and XANES. These techniques characterize and quantify S in molecular species based on “average” molecular structures rather than measuring the S content directly. In addition multi-step oxidation/reduction procedures have been combined with chromatography for the isolation of sulfur molecular species which are then characterized by sophisticated techniques such as GC/MS or FT-ICR MS. The latter technique qualitatively characterizes the molecular character after conversion of sulfides to ionizable derivatives (sulfoxides or methylsulfonium salts). A number of techniques which involve the formation of complexes with various sulfur compound classes with metal cations (Hg, Ag, Cu, and Pd) have been utilized for the separation of sulfur compounds. Historically, concentrated solutions of mercury salts were used to precipitate thiols (hence the name mercaptans) and a few lower MW sulfides from low boiling distillates. More recently, Ag(I) and Pd(II) impregnated adsorbents were used to resolve thiophenics from aromatic compounds. On 5% PdCl2 impregnated silica, aromatics are eluted with a moderate polarity solvent mixture (hexane chloroform) while with thiophenics were retained until eluted as Pd-salts with a chloroform:ether mixture. Because ether contains 2% ethanol as stabilizer, alcohol-soluble thiophene-Pd complexes were removed. Sulfide-Pd complexes are only recovered with the use of much higher concentrations of alcohol. (Both S species were “sprung” from Pd with amines.) Cation exchange resins converted to their Ag+ form have been used to separate thiophenes from aromatic hydrocarbons. In some cases, sulfides are recovered by ligand exchange displacement by excess dimethylsulfide. More recently, Ag(I) forms of cation exchangers bound to silica have been used for the same purpose. While much of the emphasis has been on the use of Pd(II) for the separation of thiophenics from aromatics, Cu(II) and Ag(I) have been used for the separation of sulfides as well. In one example of the latter, the silver forms of a strong cation exchange (SCX) silica has been incorporated into a complex HPLC separation that isolates a “sulfide” fraction from saturates, four ring sizes of aromatics and polars. In this instance, the sulfides are recovered by back-flush with an alcohol rich solvent but the distribution of sulfur is not determined.